In the semiconductor device manufacturing process, dry etching is an indispensable technique to form micropatterns. Dry etching is a method of generating a plasma in vacuum by using a reactive gas, and removing an etching target by using ions, neutral radicals, atoms, molecules, and the like in the plasma. If etching is continued after the etching target is completely removed, the underlying material may be unnecessarily etched, or the etching shape may be changed. Therefore, to obtain a structure as designed, it is very important to detect the end point of the etching process accurately.
Jpn. Pat. Appln. KOKAI Publication No. 5-102086 discloses a prior art of the end point detection method in etching, which was filed by the present inventor. According to this method, the end point of a dry etching process is determined on the basis of the intensity ratio of the emission intensity of an active species to the emission intensity of the reaction product, in the etching gas. More specifically, first, a conversion coefficient, which causes the average gradients of characteristic curves respectively representing the changes over time of the emission intensities of the active species and reaction product to coincide with each other in a predetermined specified period, is obtained. After the specified period elapses, the ratio of the emission intensity of the active species to that of the reaction product is detected while correcting the characteristic curves by using this conversion coefficient, and the end point of the dry etching process is determined on the basis of this ratio.
As described above, according to the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-102086, the two characteristic curves representing the emission intensities of the active species and reaction product after the lapse of the specified period are corrected by using the conversion coefficient that causes the average gradients of the two characteristic curves representing the emission intensities of the active species and reaction product to coincide with each other within the specified period. Since the average gradients of the characteristic curves are expressed by straight lines, i.e., linear functions, it is difficult to ideally overlay the non-linear two characteristic curves after the lapse of a specified period, as shown in FIG. 2 of this reference. Since the average gradients of the characteristic curves largely differ depending on the width of the specified period, the reliability of end point determination largely depends on the width of the specified period.
Furthermore, the method described above aims at correcting the fluctuations in emission intensities of the active species and reaction product, which are caused by a slight variation in output from the power supply, the influence of a mass-flow controller, a variation in process pressure, an increase in temperature of the target object due to the plasma, and the like, by canceling each other. However, the fluctuation components of the emission intensities of two different materials differ regardless of whether they are active species or reaction products. Accordingly, this method cannot sufficiently correct these different fluctuation components.